Ibn Ezra on Deuteronomy 4:9/אבן עזרא על דברים ד׳:ט׳

1ONLY TAKE HEED TO THYSELF. Its meaning is, should you forget everything, do not forget the day that you stood at Sinai.

2WHICH THINE EYES SAW. The reference is to And all the people perceived the thunderings, and the lightnings (Ex. 20:15).